[James T]

Good, use perl to do it then. Remember that the offending party has changed their web page to point to you and it is then joe public who connects to you, this will be many different IP addresses and impractical to block. You can perhaps log the http-referrer value and send an image that contains the text 'unapproved hot-link, please visit http://yoursite/ for legitimate link' for anything from that referrer. I can't remember the exact code required for perl to do this but the referrer property is a standard one sometimes passed from the users web browser.

Edit: or only allow it to work when your own site is the referrer.